LogSubsystem.swift

import Foundation

/// A type representing an concrete part of an application, such that it can be
/// identified in logs.
///
/// A typical use is as a nested type:
///
///     extension MyViewController {
///         enum Log: Error {
///             case user
///             case network
///         }
///         
///         @IBAction func userTappedButton(_ sender: Any) {
///             Log.user.debug("They tapped the button!")
///         }
///         
///         func requestFailed(error: Error) {
///             Log.network.error("Could not log in: %@", error)
///         }
///     }
///
protocol LogSubsystem {
    /// The name of a subsystem, such as "com.myapp.networking".
    ///
    /// By default, the fully-qualified name of `self`.
    static var name: String { get }
    
    /// A stage or grouping for a subsystem, such as "setup" or "teardown".
    ///
    /// By default, the description of `self`.
    var categoryName: String { get }

    /// Whether to print `debug` messages.
    static var isDebugEnabled: Bool { get }

    /// Whether to print `info` messages.
    static var isInfoEnabled: Bool { get }
}

/// A type with a customized log representation.
///
/// Types that conform to the `CustomLogStringConvertible` protocol can provide
/// their own representation to be used when logging using a `LogSubsystem`.
///
/// Accessing a type's `logDescription` property directly is discouraged.
protocol CustomLogStringConvertible {
    /// A console log representation of `self`.
    var logDescription: String { get }
}

extension String {

    /// Creates a console log representing the given value.
    init<Subject>(logging value: Subject) {
        if let subject = value as? CustomLogStringConvertible {
            self = String(describing: subject.logDescription)
        } else {
            self = String(describing: value)
        }
    }
    
}

extension LogSubsystem {
    
    static var name: String {
        return String(reflecting: self)
    }
    
    var categoryName: String {
        return String(describing: self)
    }

    static var isDebugEnabled: Bool {
        return false
    }

    static var isInfoEnabled: Bool {
        return false
    }
    
}

extension LogSubsystem {

    /// Logging shim. This was intended to be backed by `os_log`, but as of
    /// Xcode 8.2 that unconditionally mirrors to `stderr`, which completely
    /// defeats the point.
    ///
    /// When a future Xcode fixes this (probably the next one, see link below),
    /// the prototypes below could be changed to something a la:
    ///
    ///     func log(_ message: StaticString, dso: UnsafeRawPointer?, level: OSLogType, _ arguments: [Any])
    ///
    /// And encoding the `os_log` buffer from there.
    ///
    /// - see: https://github.com/apple/swift-lldb/blob/swift-3.1-branch/docs/structured_data/DarwinLog.md
    private func log(_ message: StaticString, _ arguments: [Any]) {
        let cArguments = arguments.map { (argument) -> CVarArg in
            switch argument {
            case let value as CustomLogStringConvertible:
                return value.logDescription as NSString
            case let value as CVarArg:
                return value
            default:
                return String(describing: argument) as NSString
            }
        }

        withVaList(cArguments) {
            NSLogv("[\(categoryName)] \(message)", $0)
        }
    }
    
    func debug(_ message: StaticString, _ arguments: Any...) {
        guard Self.isDebugEnabled else { return }
        log(message, arguments)
    }

    func info(_ message: StaticString, _ arguments: Any...) {
        guard Self.isInfoEnabled else { return }
        log(message, arguments)
    }

    func show(_ message: StaticString, _ arguments: Any...) {
        log(message, arguments)
    }
    
    func error(_ message: StaticString, _ arguments: Any...) {
        log(message, arguments)
    }
    
    func fault(_ message: StaticString, _ arguments: Any...) {
        log(message, arguments)
    }
    
}